1. Field of the Invention
The present invention relates to an exhaust system that exhausts exhaust gas from an engine, and an engine device and a vehicle with the same.
2. Description of the Background Art
Conventionally, a catalyst device has been provided in an exhaust system to remove harmful substances contained in the exhaust gas emitted from an engine.
In order to quickly activate the catalyst device, the temperature of the catalyst needs to be rapidly increased in a short period of time. Consequently, there has been developed an exhaust system in which the catalyst device is arranged closer to the engine, so that high temperature exhaust gas flows into the catalyst.
For example, in an exhaust system with a catalyst in a motorcycle described in JP 3242488 B, auxiliary oxidation catalysts are provided in a front exhaust pipe and a rear exhaust pipe connected to a front cylinder and a rear cylinder of a V-type two cylinder engine. However, in the configuration described in JP 3242488 B, the auxiliary oxidation catalysts need to be increased in number with an increase in the number of the cylinders of the engine, which increases the manufacturing cost.
As a method to solve such a problem, there is a method in which exhaust pipes of a plurality of cylinders are merged into one exhaust pipe and a catalyst device is provided at this portion where the exhaust pipes are merged (e.g., JP 2001-241323 A and JP 2002-309930 A). This can reduce the number of catalyst devices to be installed.
In an exhaust treatment device of the exhaust gas of an internal combustion engine described in JP 2001-241323 A, exhaust pipes of first and third cylinders, in the order of ignition, are merged into one chamber to cause the exhaust gas to flow into a catalyst device through this chamber. Furthermore, exhaust pipes of second and fourth cylinders, in the order of ignition, are merged into the other chamber to cause the exhaust gas to flow into a catalyst device from this chamber.
In an integral dual catalyst exhaust manifold described in JP 2002-309930 A, exhaust pipes coupled to a first cylinder and a fourth cylinder are merged to connect to a first catalyst container, and exhaust pipes coupled to a second cylinder and a third cylinder are merged to connect to a second catalyst container.
Meanwhile, when the exhaust pipes of the plurality of cylinders are merged into one exhaust pipe, exhaust gas pressure interference occurs at the merging portion. The output performance of the engine is largely influenced by a length of the exhaust pipes to the merging portion where this exhaust gas pressure interference occurs. FIG. 10 is a chart for explaining a change in the output characteristics of an engine resulting from varying the exhaust pipe length to the merging portion. In FIG. 10, the horizontal axis indicates the engine speed and the vertical axis indicates a generated torque of the engine. Furthermore, the dotted line indicates an output characteristic of the engine in the case where the exhaust pipe length leading to the merging portion is long, and the solid line indicates an output characteristic of the engine in the case where the exhaust pipe length leading to the merging portion is short.
Generally, in the case where the length of the exhaust pipes leading to the merging portion is long, the generated torque of the engine becomes highest in a region where the engine speed is low as shown in FIG. 10. On the other hand, in the case where the length of the exhaust pipes leading to the merging portion is short, the generated torque of the engine becomes highest in a region where the engine speed is high as shown in FIG. 10. That is, in such a vehicle where it is desirable to have a large generated torque of the engine at a low speed, the length of the exhaust pipes leading to the merging portion is preferably long, while in such a vehicle where it is desirable to have a large generated torque of the engine at a high speed, the length of the exhaust pipes leading to the merging portion is preferably short.
However, in the configurations of the exhaust pipes described in the above-described JP 2001-241323 A and JP 2002-309930 A, if the length leading to the merging portion of each of the exhaust pipes becomes long, a distance between the engine and the catalyst becomes long, so that a temperature of the exhaust gas decreases before the exhaust gas reaches the catalyst. In this case, the catalyst cannot be quickly activated.